Modification of Exploration of Long‐term Nutrient Trajectories for Phosphorus (ELEMeNT-P) model to quantify legacy phosphorus dynamics in a typical watershed of eastern China

Excess anthropogenic phosphorus (P) inputs lead to the accumulation of legacy P in watersheds, which subsequently becomes a persistent source of P pollution to aquatic systems thereby hindering the effectiveness of water quality mitigation strategies. The ELEMeNT-P (Exploration of Long-tErM Nutrient Trajectories-Phosphorus) model is a process-based model for simulating legacy P dynamics that conceptualizes the hydrological and biogeochemical legacy components of P inputs to riverine export. However, ELEMeNT-P adopts a soil erosion loss equation originally designed for the Mediterranean region and ignores the threshold transition of soil P leaching that occurs following P sorption saturation, making it less applicable to other areas. Herein, we applied the Chinese soil loss equation to estimate soil erosion and an exponential function based on peak flow to calculate surface runoff P loss in a Chinses watershed. Further, a breakthrough curve was incorporated to characterize the rapid increase in soil P leaching once the soil P content exceeded a specified threshold. The modified ELEMeNT-P model was then evaluated for the Yongan watershed in eastern China to quantify legacy P dynamics over a 41-year period (1980–2020). The modified model improved the efficiency metrics for riverine P fluxes compared to the original model (R²: 0.94 vs. 0.79 and Nash-Sutcliffe Efficiency: 0.94 vs. 0.48). The P saturation ratio of 50 % for soils exceeded the threshold, indicating a high risk of P leaching, while Hedley P fractionation and soil total P test validated the model’s results in soil P type proportions and spatial distributions of legacy P (R² = 0.56). The results revealed that legacy P accumulation showed considerable spatial heterogeneity, with 40.0–504.9 kg P ha⁻¹ accumulated in soil and 26.7–41.5 kg P ha⁻¹ accumulated in groundwater. Riverine P flux mainly originated from legacy P pools (60.1 %, range: 21.5–90.3 %). The recovery time for depletion of the legacy P stock to the 1980 level via riverine export was 300 years, assuming total cessation of P inputs. Overall, the modified model was effective for determining the spatial and temporal distribution of legacy P within the watershed and therefore serves as an effective method to guide watershed P management.